Information recording/reproducing device

ABSTRACT

An information recording/reproducing device includes a recording layer, and a recording circuit which records data to the recording layer by generating a phase change in the recording layer. The recording layer includes a first chemical compound having a spinel structure. The recording layer is A x M y X 4  (0.1≦x≦2.2, 1.0≦y≦2.0), where A includes one selected from a group of Zn, Cd and Hg, M includes one selected from a group of Cr, Mo, W, Mn and Re, and X includes O.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of PCT Application No.PCT/JP2008/060562, filed Jun. 9, 2008, which was published under PCTArticle 21(2) in Japanese.

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2007-155677, filed Jun. 12, 2007,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an information recording/reproducingdevice with a high recording density.

2. Description of the Related Art

In recent years, compact portable devices have been widely usedworldwide and, at the same time, a demand for a small-sized andlarge-capacity nonvolatile memory has been expanding rapidly along withthe extensive progress of a high-speed information transmission network.Among them, particularly a NAND type flash memory and a small-sized HDD(hard disk drive) have rapidly evolved in recording density, andaccordingly, they now form a large market.

On the contrary, some ideas for a new memory have been proposed, withthe goal of greatly increasing the limit of recording density.

For instance, PRAM (phase change memory) adopts a principle in whichmaterials capable of taking two conditions, an amorphous condition (OFF)and crystalline condition (ON), are used as recording materials, andthese two conditions are caused to correspond to binary data “0” and “1”to record data.

Write/erase is performed in such a way that, for instance, the amorphouscondition is prepared by applying a large power pulse to the recordingmaterial, and the crystalline condition is prepared by applying a smallpower pulse to the recording material.

A read is performed by causing a small read current to flow in therecording material to the degree that the write/erase is not generated,followed by measuring an electric resistance of the recording material.The resistance value of the recording material in the amorphouscondition is larger than the resistance value of the recording materialin the crystalline condition, and its ratio is in the degree of 10³.

The greatest feature of the PRAM lies in a point that, even thoughelement size is reduced to about 10 nm, the element can be operated. Inthis case, since the recording density of about 10 Tbpsi (tera bytes persquare inch) can be realized, and accordingly, this is one of candidatesfor realizing increased recording density (for instance, refer to JP-A2005-252068 (KOKAI)).

Further, a new memory has been reported which is different from the PRAMbut has a very similar operation principle to the PRAM (for instance,refer to JP-A 2004-234707 (KOKAI)).

According to this report, a representative example of a recordingmaterial to record data is nickel oxide, in which, like the PRAM, thelarge power pulse and the small power pulse are used for performing thewrite/erase. There has been reported an advantage that the powerconsumption at the time of the write/erase becomes small as comparedwith the PRAM.

Until now, although the details of an operation mechanism of the newmemory are not clear, reproducibility is confirmed, and thus this isnoticed as one of the candidates for the increased recording density.Further, some research groups are attempting to clarify the operationmechanism.

In addition thereto, proposed is a MEMS memory using MEMS (micro electromechanical system) technology (for instance, refer to P. Vettiger, G.Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M.A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binng, IEEE Trans.Nanotechnology 1, 39 (2002)).

In particular, the MEMS memory, called Millipede, has a structure inwhich array shaped cantilevers face a recording medium to which anorganic substance is applied, and a probe at a tip of the cantilevercomes into contact with the recording medium with appropriate pressure.

A write is performed by selectively controlling the temperature of aheater added to the probe. That is, when increasing the temperature ofthe heater, the recording medium is softened, and then, depressions areformed on the recording medium because the probe forms dents in therecording medium.

A read is performed by scanning the probe on a surface of the recordingmedium while causing a current to flow through the probe to the degreethat the recording medium is not softened. When the probe sinks into thedepression of the recording medium, temperature of the probe decreases,and the resistance value of the heater increases, so that it is possibleto sense the data by reading this change of resistance value.

The greatest feature of the MEMS memory such as the Millipede lies in apoint that since it is not necessary for each recording part to providewiring to record bit data, the recording density can be improvedremarkably. Under existing circumstances, a recording density of about 1Tbps has already been achieved (for instance, refer to P. Vettiger, T.Albrecht, M. Despond, U. Drechsler, U. Durig, B. Gotsmann, D. Jubin, W.Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesmann and G. K.Binng, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A.Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDM03 pp.763 to 766).

Further, subsequent to the Millipede, recently, performed are attemptsto achieve a large improvement concerning power consumption, recordingdensity or working speed while combining the MEMS technique with a newrecording principle.

For instance, proposed is a system in which a ferroelectric layer isprovided on the recording medium, and recording of the data is performedby causing dielectric polarization in the ferroelectric layer byapplying a voltage to the recording medium. Theoretically, this systemis predicted to be able to utilize one crystal as a unit (recordingminimum unit) for recording one byte of data.

If the recording minimum unit is equivalent to one unit cell of thecrystal of the ferroelectric layer, the recording density rises to aphenomenal approx. 4 Pbpsi (peta bytes per square inch).

The MEMS memory which stores data by using the ferroelectric layer isoperated by a well-known principle, but the MEMS memory is not realizedat the present time.

One of the biggest reasons is that the electric field from the recordingmedium is shielded by ions in an air. In other words, reading does notexecute, because the electric field from the recording medium is notsensed.

The other reason is that a lattice defect in a crystal shields charges,because charges caused by the lattice defect are transferred to therecording part.

About the problem of the electric shield by ions in an air, recently,based on development of a read system using SNDM (scanning nonlineardielectric microscope), the new memory has advanced considerably towardpractical use (for instance, refer to A. Onoue, S. Hashimoto, Y. Chu,Mat. Sci. Eng. B120, 130 (2005)).

BRIEF SUMMARY OF THE INVENTION

The present invention proposes a nonvolatile informationrecording/reproducing device with high recording density and low powerconsumption.

The inventors conduct research on a resistance change phenomenon inoxides and find that diffusion of a cation in the oxide and an attendantchange of a valence of an ion contribute to the resistance changephenomenon.

That is, in order to cause the resistance change with small powerconsumption, it is only necessary to facilitate the diffusion of thecation. Therefore, the invention has a configuration in which alarge-size diffusion path is provided in order to cause the resistancechange with small power consumption.

In accordance with an aspect of the invention, an informationrecording/reproducing device includes a recording layer, and means forrecording information to the recording layer by causing a phase changein the recording layer by applying voltage to the recording layer,wherein the information recording/reproducing device includes a firstchemical compound having a spinel structure, which is represented byChemical Formula 1: A_(X)M_(Y)X₄ (0.1≦x≦2.2, 1.0≦y≦2), where A includesat least one element selected from a group of Zn, Cd, and Hg, M includesat least one transition element selected from a group of Cr, Mo, W, Mnand Re, and X includes O.

In the information recording/reproducing device in accordance with anaspect of the invention including a recording layer, and means forrecording information to the recording layer by causing a phase changein the recording layer by applying voltage to the recording layer, therecording layer includes a first chemical compound having the spinelstructure and a second chemical compound having a vacant site which acation element in the spinel structure can occupy.

Accordingly, the nonvolatile information recording/reproducing devicewith high recording density and low power consumption can be implementedin the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1 to 3 are views, each showing a recording principle.

FIG. 4 is a view showing a probe memory.

FIG. 5 is a view showing a recording medium.

FIG. 6 is a view showing the condition of recording.

FIG. 7 is a view showing a write operation.

FIG. 8 is a view showing a read operation.

FIG. 9 is a view showing a write operation.

FIG. 10 is a view showing a read operation.

FIG. 11 is a view showing a semiconductor memory.

FIG. 12 is a view showing a memory cell array.

FIG. 13 is a view showing a memory cell.

FIGS. 14 and 15 are views, each showing a memory cell array.

FIG. 16 is a view showing an application example for a flash memory.

FIGS. 17 to 20 are views, each showing a NAND cell unit.

FIGS. 21 and 22 are views, each showing a NOR cell.

FIGS. 23 to 25 are views, each showing a 2-transistor cell unit.

FIG. 26 is a view showing an inversion in a spinel structure.

FIG. 27 is a view showing a recording principle.

FIGS. 28 and 29 are views, each showing an example of a memory cellarray structure.

FIGS. 30 and 31 are views, each showing a modified example of arecording layer.

DETAILED DESCRIPTION OF THE INVENTION

An information recording/reproducing device of an aspect of the presentinvention will be described below in detail with reference to theaccompanying drawing.

1. OUTLINE

(1) An information recording/reproducing device according to a firstexample of the invention includes a recording layer including at least afirst chemical compound having a spinel structure which is representedby Chemical Formula 1: A_(X)M_(Y)X₄ (0.1≦x≦2.2, 1.0≦y≦2), where Aincludes at least one element selected from a group of Zn, Cd, and Hg, Mincludes at least one transition element selected from a group of Cr,Mo, W, Mn and Re, and X includes O.

As to molar ratios x and y, a lower limit of a numeric range is set inorder to maintain a crystal structure, and an upper limit thereof is setin order to control an electron state in a crystal.

The use of these elements enlarges the diffusion path of the A ion,which allows realization of the low power consumption.

The use of such recording layer can realize the Pbpsi-level recordingdensity in principle and also achieve the low power consumption.

(2) An information recording/reproducing device according to a secondexample of the invention has a word line and a recording layer having aspinel structure, and records data by a phase change of the recordinglayer generating by a voltage which applies the recording layer. Therecording layer includes a first chemical compound having a spinelstructure and a second chemical compound having a vacant site which acation element can occupy.

The second chemical compound is one of:

_(x)M2X2₂  Chemical Formula 2

where

is a vacant site which the cation element can occupy, M2 is at least oneelement selected from Ti, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W,Re, Ru, and Rh, X2 is at least one element selected from O, S, Se, N,Cl, Br, and I, and 0.3≦x≦1;

_(x)M2X2₃  Chemical Formula 3

where

is a vacant site which the cation element can occupy, M2 is at least oneelement selected from Ti, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W,Re, Ru, and Rh, X2 is at least one element selected from O, S, Se, N,Cl, Br, and I, and 1≦x≦2;

_(x)M2X3₄  Chemical Formula 4

where

is a vacant site which the cation element can occupy, M2 is at least oneelement selected from

Ti, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, and Rh, X3 isat least one element selected from O, S, Se, N, Cl, Br, and I, and1≦x≦2;

_(x)M2PO_(z)  Chemical Formula 5

where

is a vacant site which the cation element can occupy, M2 is at least oneelement selected from Ti, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W,Re, Ru, and Rh, P is phosphorus, O is oxygen, 0.3≦x≦3, and 4≦z≦6; and

_(x)M2O₅  Chemical Formula 6

where

is a vacant site which the cation element can occupy, M2 is at least oneelement selected from V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, andRh, O is oxygen, and 0.3≦x≦2.

In Chemical Formulae 2 to 6,

is a vacant site which A element can occupy. Alternatively, other ionsmay occupy a part of the vacant sites in order to facilitate filmdeposition of the second chemical compound 12B.

The second chemical compound has one of crystal structures such as ahollandite structure, a ramsdellite structure, an anatase structure, abrookite structure, a pyrolusite structure, a ReO₃ structure, aMoO_(1.5)PO₄ structure, a Ti_(0.5)PO₄ structure, an FePO₄ structure, aβMnO₂ structure, a γMnO₂ structure, and a λMnO₂ structure.

A Fermi level of an electron of the first chemical

compound is set lower than a Fermi level of an electron of the secondchemical compound. This is one of necessary conditions thatreversibility is imparted to the state of the recording layer. At thispoint, a value measured from a vacuum level is used as each Fermi level.

It is preferable that a coincidence of lattice constant between thefirst and second chemical compounds is improved to orient the secondchemical compound properly, when the second chemical compound comprisesa material made of a spinel structure having many vacant sites.

The structure of crystal is distorted from a cubic crystal, when x ofZnMn_(2-x)Ms_(x)O₄ (where Ms is a displacement element) is small.Therefore, it is preferable that the coincidence of lattice constantbetween the first and second chemical compounds is improved, when amaterial made of a hollandite structure is used as the second chemicalcompound.

The use of the above-described recording layer can realize thePbpsi-level recording density in principle and also achieve the lowpower consumption.

2. BASIC PRINCIPLE OF RECORDING/ERASING/REPRODUCING

(1) A basic principle of recording/erasing/reproducing of data in theinformation recording/reproducing device of the first example of theinvention will be described.

FIG. 1 shows a structure of a recording unit.

In FIG. 1, the reference number 11 denotes an electrode layer, thereference number 12 denotes a recording layer, and the reference number13A denotes an electrode layer (or protection layer).

A small white circle in the recording layer 12 expresses a diffusionion, and a large white circle expresses an anion (X ion). A small blackcircle expresses a transition element ion. Typically the diffusion ioncorresponds to the A ion, and the transition element ion corresponds tothe M ion.

A part of the diffusion ions transfers in the crystal, when a voltage isapplied to the recording layer 12 to generate a potential gradient inthe recording layer 12. Therefore, in the first example, an initialstate of the recording layer 12 is set to an insulating material(high-resistance state), and the phase change of the recording layer 12is caused to impart conductivity (low-resistance state) to the recordinglayer 12 by the potential gradient, thereby performing the recording.

First, for example, a potential at the electrode layer 13A is setrelatively lower than a potential at the electrode layer 11. A negativepotential may be imparted to the electrode layer 13A when the electrodelayer 11 is set to a fixed potential (for example, ground potential).

At this point, a part of the diffusion ions in the recording layer 12transfers onto the side of the electrode layer (negative electrode) 13A,and the number of diffusion ions in the recording layer (crystal) 12 isrelatively decreased with respect to the number of anions. The diffusionions which transfer onto the side of the electrode layer 13A receiveelectrons from the electrode layer 13A, and the diffusion ions aredeposited as metal, thereby forming a metallic layer 14.

Alternatively, for example, when the vacant site which the diffusion ioncan occupy exists in the crystal structure of the recording layer 12like the spinel structure, the vacant site on the side of the electrodelayer 13A may be filled with the diffusion ion which transfers onto theside of the electrode layer 13A. In such cases, in order to satisfy aneutral condition of a local charge, the diffusion ion receives theelectron from the electrode layer 13A to act as metal.

The number of anions becomes excessive in the recording layer 12, whichincreases the valence of the transition element ion in the recordinglayer 12. That is, because the recording layer 12 acquires the electronconductivity by the injection of the carrier, the recording (setoperation) is completed.

A current pulse is passed through the recording layer 12 to detect theresistance of the recording layer 12, thereby easily performing thereproducing. However, it is necessary to set the current pulse to aminute value of an extent that a material constituting the recordinglayer 12 does not cause the resistance change.

The above-described process is a kind of electrolysis, and it can beconsidered that an oxidant is generated on the side of the electrodelayer (positive electrode) 11 by electrochemical oxidation while areducing agent is generated on the side of the electrode layer (negativeelectrode) 13A by electrochemical reduction.

Therefore, in order to return the recording state (low-resistance state)to the initial state (high-resistance state), for example, Joule heatingof the recording layer 12 may be performed by a large current pulse topromote an oxidation-reduction reaction of the recording layer 12. Thatis, the recording layer 12 is returned to the insulating material byresidual heat after the large current pulse is cut off (resetoperation).

Alternatively, the reset operation may be performed by applying anelectric field opposite to that of the set operation. That is, as withthe set operation, when the electrode layer 11 is set to the fixedpotential, the positive potential may be imparted to the electrode layer13A. Therefore, in addition to the oxidation-reduction reaction by theJoule heat, the metallic layer is oxidized near the electrode layer 13Ato become the cation, and the cation is returned into the matrixstructure by a potential gradient in the recording layer 12. Because thevalence is decreased to the same value as the pre-set operation in thetransition element ion whose valence is increased, the recording layer12 is returned to the initial insulating material.

However, in order to put the operation principle into practical use, itis necessary to confirm that the reset operation is not caused at roomtemperature (securement of sufficiently long retention time) and thatthe power consumption is sufficiently small in the reset operation.

The problem with the securement of the sufficiently long retention timecan be solved by imparting the valence of at least two to the diffusionion.

When the diffusion ion has the valence of one like a Li ion, asufficient ion transfer resistance is not obtained in the set state, thediffusion ion element is instantaneously returned from the metalliclayer 14 into the recording layer 12. In other words, the sufficientlylong retention time is not obtained.

When the diffusion ion has the valence of at least three, because thevoltage necessary for the set operation is increased, possibly collapseof the crystal is caused in the worst case.

Accordingly, preferably the diffusion ion (A ion) has the valence of twoin the information recording/reproducing device.

The problem with the sufficiently small power consumption in the resetoperation can be solved by a structure having the transfer path, inwhich an ion radius of the diffusion ion is optimized such that thediffusion ion can transfer in the recording layer (crystal) 12 withoutcausing the collapse of the crystal. The above-described elements andcrystal structures may be used as such recording layer 12.

Particularly, it is well known that the cation transfer easily in thespinel structure, so that preferably the spinel structure may be used asthe recording layer 12. However, even in such spinel structure, it isnecessary to optimize a combination of the A ion and the M ion in orderto cause repeatedly stable switching (ion transfer).

An example, in which the A ion corresponds to the diffusion ion of FIG.1 while the M ion corresponds to the transition element ion of FIG. 1 inthe spinel structure expressed by Chemical Formula AM₂X₄, will bedescribed below.

In the spinel structure, a phenomenon (inversion) in which a site wherethe A ion exists and a site where the M ion exists are replaced witheach other is reported as shown in FIG. 26, and a degree of thephenomenon is referred to as inversion parameter.

The large inversion parameter means that a probability of existence ofthe M ion in the transfer path of the A ion is increased from theviewpoint of the whole of the crystal, and the large inversion parametermeans that the A ion and the M ion are easily replaced with each otherat a particular site from the local viewpoint.

The inversion is easily caused when the crystal is exposed to a hightemperature. Therefore, the inversion is easily caused not only when thefilm is deposited, but also when the recording layer is heated by Jouleheat in the set and reset operations.

Because the A ion differs from the M ion in an ion radius and bondstrength with the anion (X ion), a strain is generated in a latticeevery time the A ion and the M ion are replaced with each other, and aprobability of collapse of the crystal structure is increased.

Accordingly, in order to repeatedly cause the stable phase change,preferably a combination of the A ion and the M ion is selected suchthat the inversion parameter becomes zero.

Possibly the inversion parameter which is not zero causes thereplacement of the position between the A ion and the M ion when therecording layer is retained for a long time in the high-resistance-statephase or the low-resistance-state phase.

The spinel conductivity largely depends on a hybrid orbital formed bythe M ion and the X ion. Therefore, unfortunately the resistance ischanged in each phase when the inversion is caused, that is, thermalstability of the resistance of the recording layer is lowered when theinversion is increased.

Generally a diffusion coefficient (ease of transfer) of the ion in thecrystal depends on ion species, and energy required for anoxidation-reduction reaction also depends on ion species. Accordingly,when the inversion is caused, a degree in which the A ion transferslocally fluctuates, which causes a problem in that the resistancefluctuates in each resistance-state phase.

That is, in order to repeatedly cause the stable resistance change, thecombination of the A ion and the M ion may be selected such that theinversion parameter becomes zero.

When oxygen O is used as X, it is well known that a IIb-group element ispreferably used as the A ion in order to decrease the inversion.Accordingly, preferably Zn, Cd, and Hg are used as the A ion.

On the other hand, preferably Va-group, VIa-group, and VIIa-groupelements are used as the M ion which forms the matrix structure alongwith the X ion. The A ion transfers in the matrix structure.

In the above-described combination, the inversion parameter becomessubstantially zero. However, it is reported that the inversion parameterdoes not become zero in ZnFe₂O₄, MgCr₂O₄, and the like except for theabove-described combination.

In order to sufficiently secure the ion mobility, preferably Zn havingthe small ion radius is used as the A ion. An influence of the inversionis increased, as an element size is reduced, and as a film thickness ofthe recording layer is decreased.

On the other hand, the following items can be cited as the conditionsuitable for the atomic species used as the M ion.

The first is that the M ion is in the low-resistance state when the Aion is drawn, and the second is that the M ion stably retains thestructure after the A ion is drawn.

When the structure is not stably retained after the A ion istransferred, because the site to which the A ion should be returned iseliminated, the set and reset operations cannot stably be repeated.

In order that the structure in which the A ion is drawn is stable, andin order that the Joule heat generated in the set and reset operationsdoes not largely influence the matrix structure, the stable existence ofthe chemical compound expressed by Chemical Formula M₂X₄ (equal to MX₂)may preferably satisfy the neutral condition of the local charge.

Preferably the conductive crystal expressed by the Chemical Formula MO₂exists, when oxygen O is used as X while the VIa-group and VIIa-groupelements are used as the M ion. VO₂ in which Va-group atom is usedexhibits a metal-insulating material transition near room temperature,and possibly a property of VO₂ varies according to an operatingtemperature.

In order to repeatedly cause the stable switching from theabove-described discussion, preferably Zn, Cd, and Hg are used as the Aion element while Cr, Mo, W, Mn, and Re are used as a principalcomponent ion (Mm ion) element of the M ion such that the smallinversion parameter and thermally-stable matrix structure are obtained.

A more preferred example of the atomic species used as the M ion willfurther be described below.

Even if the conductive crystal exists stably in the form of MO₂, becausea distance between the M ion and the O ion is changed by the valence ofthe M ion, the crystal lattice of the recording layer 12 is slightlydeformed when the set and reset operations are performed.

When the MO₂ exists as the crystal having the λMnO₂ structure which isof the structure in which the A ion is drawn from the spinel structure,the crystal structure is easily kept constant irrespective of theexistence of the A ion.

Accordingly, more preferably the VIIa-group element is used as the Mion. It is well known that the VIIa-group element has the small relativeion radius change caused by the valence change, and it is well knownthat the VIIa-group element forms the crystal having the λMnO₂structure.

That is, more preferably Mn and Re are used as the Mm ion element. Inconsideration of the ease of control of the electron state in thecrystal, most preferably Mn having the small ion radius is used as theMm ion element.

In a more preferable configuration, it is well known that a part of theMm ion elements may be replaced with a substituent ion (Ms ion) elementin order to buffer the deformation of the lattice.

For example, it is well known that Zn, Cr, Fe, Co, Ni, Al, and Ga aresuitably used as the Ms ion element for the above-described purpose.

In cases where Al which is not the transition element is used as the Msion element, the neutral condition of the charge is not satisfied nearAl when the A ion adjacent to Al transfers.

Accordingly, the transfer of the A ion is hardly caused in theneighborhood of Al, and therefore the change in lattice distance ishardly caused in the neighborhood of Al. Therefore, the deformation ofthe whole recording layer can be prevented.

The collapse of the crystal structure in the recording layer due to thephase change is avoided by preventing the deformation of the wholerecording layer, so that further improvement of the repetitionresistance property can be expected. When Mn is used as the Mm ionelement, most preferably Al is used as the Ms ion element because Mn andAl have the ion radii which are close to each other.

Finally an optimum value of a mixture ratio of the atoms will bedescribed.

There is a slightly arbitrary property in the mixture ratio of the Aion, when the vacant site which the A ion can occupy exists, or when theA ion can occupy the site which the M ion fundamentally occupies.

Accordingly, the mixture ratio of the A ion is set in a range of0.1≦x≦2.2. Actually the mixture ratio of the A ion can be optimized suchthat the resistance of each state or the diffusion coefficient of the Aion becomes an optimum value.

When the mixture ratio of the A ion is excessively small, it isdifficult to stably produce and retain the structure. When the mixtureratio of the A ion is excessively large, it is difficult to diffuse theA ion. Accordingly, more preferably the mixture ratio of the A ion isset in the range of 0.5≦x≦1.5.

When the mixture ratio of the M ion (Mm ion and Ms ion) exceeds two, itis necessary to locate the M ion in the site which the A ion canfundamentally occupy, thereby it is difficult to diffuse the A ion.

On the other hand, when the total amount of ion which can occupy thesite of the M ion is excessively small, it is difficult to stably retainthe structure after the A ion is drawn.

Accordingly, preferably the mixture ratio of the M ion is set in therange of 1.5≦y≦2. As described later, more preferably the mixture ratioof the M ion is set in the range of 1.8≦y≦2 except that the A ion canoccupy the site of the M ion.

A substitution amount of Ms ion=(Ms ion)/(Mm ion+Ms ion) will bedescribed below.

When the amount of Ms ion exceeds the mixture amount of Mm ion, theeffect that the Mm ion is selected such that the stable switching canrepeatedly be performed is weakened, preferably the substitution amountis set to 0.5 or less.

When the Ms ion does not evenly disperse in the M ion, because the easeof transfer of the A ion depends on the position to generate adifference in resistance, it is necessary that the Ms ion disperseevenly in the M ion. Therefore, preferably the substitution amount issubstantially lower than 0.4 or less.

On the other hand, when the mixture amount of Ms ion is excessivelydecreased, the effect of the addition of the Ms ion is insufficientlyobtained. Accordingly, when the Ms ion is intentionally used, morepreferably the substitution amount is not lower than 0.1.

Therefore, preferably the substitution amount is set in the range ofsubstitution amount ≦0.5, and more preferably the substitution amount isset in the range of 0.1≦substitution amount ≦0.4.

All of the A ions do not transfer unlike the case in which the crystalis used as a secondary battery. Therefore, when the A ion and the Mm ionare used with the suitable combination, the recording layer in which thestable resistance change is repeatedly performed even if the Ms ion isnot used can be obtained by optimizing the voltage pulse condition andthe like.

For example, when Zn is used as the A ion element while Mn is used asthe Mm ion element, the Zn ion can occupy the site of the M ion toobtain the substitution effect in the mixture ratio in which the sitesof M ions are incompletely filled with the Mn ions because the Mn ratiois slightly lower than two.

Accordingly, when Zn is used as the A ion element while Mn is used asthe Mm ion element, the effect that the deformation of the lattice isbuffered can be imparted without use of the substituent ion by adjustingthe mixture ratio.

The case in which the sufficiently large crystal is obtained isdescribed in FIG. 1. On the other hand, even if the crystal is disposedwhile divided in a film thickness direction as shown in FIG. 27, thetransfer of the A ion and the corresponding resistance change are causedby the mechanism described in the invention.

That is, when the negative voltage is applied to the electrode layer 13while the electrode layer 11 is grounded, the potential gradient isgenerated in the recording layer 12 to transport the A ion. When the Aion transfers to a crystal interface, the A ion gradually receives theelectron from the region close to the electrode layer 13A, and the A ionbehaves as metal. As a result, the metallic layer 14 is formed near thecrystal interface.

The conductivity of the recording layer 12 increases because the valenceof the transition element ion increases in the recording layer 12. Insuch cases, a conductive path of the metallic layer is formed along thecrystal interface, the resistance between the electrode layer 11 and theelectrode layer 13 decreases, and the element is changed to alow-resistance-state phase.

In this case, the A ion at the crystal interface is returned into thespinel structure by the Joule heating using the large current pulse orthe application of the reversed voltage pulse, which allows thelow-resistance-state phase to be changed to the high-resistance-statephase.

However, in order that the transfer of the diffusion ion is efficientlycaused to the applied voltage as shown in FIG. 1, preferably thedirection in which the diffusion ion diffuses and the direction in whichthe electric field is applied are matched with each other.

In the spinel structure, as shown in FIG. 1, when the c-axis of therecording layer is orientated in parallel with the film surface,preferably the transfer path is disposed in the direction in which theelectrodes are connected. When a [110] direction of the recording layeris orientated substantially perpendicular to the recording layer, thetransfer path is disposed in substantially parallel with the electricfield. Therefore, more preferably the recording layer is substantiallyorientated toward a (110) direction.

The inside of the crystal structure differs from a circumferentialportion of the crystal grain in the ease of ion transfer. Therefore, inorder to utilize the transfer of the diffusion ion in the crystalstructure to equalize recording/erasing property in different positions,preferably the recording layer includes a polycrystalline state or asingle crystal state. When the recording layer is in the polycrystallinestate, in consideration of the ease of the film deposition, preferably asize in a sectional direction of the recording film of the crystal grainfollows a distribution having a single peak, and an average of thecrystal grain size is 3 nm or more. Preferably the average of thecrystal grain size is not lower than 5 nm because the film is depositedmore easily, and more preferably the average of the crystal grain sizeis not lower than 10 nm because the recording/erasing property canfurther be equalized in different positions.

The film thickness of the recording layer may appropriately be set suchthat the resistances of the high-resistance-state phase andlow-resistance-state phase become desired values. Typically the filmthickness of the recording layer ranges from 1 to 500 nm. When arecording region is reduced, preferably the film thickness of therecording layer is smaller than ten times the recording region in orderto suppress the expansion of the recording layer in an in-planedirection.

Because the oxidant is generated on the side of the electrode layer(positive electrode) 11 after the set operation, preferably theelectrode layer 11 is made of a material (such as electric conductivenitride and electric conductive oxide) which is hardly oxidized.

Preferably the material used for the electrode layer 11 does not haveion conductivity.

The materials used for the electrode layer 11 are listed below. Amongothers, LaNiO₃ is most suitable from the viewpoint of overallperformance including the good electric conductivity.

MN

M is at least one element selected from the group of Ti, Zr, Hf, V, Nb,and Ta, and N is nitrogen.

MO_(X)

M is at least one element selected from the group of Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Ir, Os, and Pt.It is assumed that the molar ratio x satisfies 1≦x≦4.

AMO₃

A is at least one element selected from the group of La, K, Ca, Sr, Ba,and Ln [Lanthanide].

M is at least one element selected from the group of Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Ir, Os, and Pt,and O is oxygen.

A₂MO₄

A is at least one element selected from the group of K, Ca, Sr, Ba, andLn [Lanthanide].

M is at least one element selected from the group of Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Ir, Os, and Pt,and O is oxygen.

The reducing agent is generated on the side of the protection layer(negative electrode) 13 after the set operation, preferably theprotection layer 13 has a function of preventing the recording layer 12from reacting with the atmosphere.

Alternatively, a buffer layer may be provided between the recordinglayer and the electrode layer 11 in order to control the orientation ofthe recording layer. Oxides of Ir or Ru and nitrides of Si, Ti, Zr, Hf,V, Nb, Ta, and W may be cited as an example of a material suitable forthe buffer layer. In addition, preferably the buffer layer is orientatedso as to become an integral multiple of a lattice constant of therecording layer orientated toward the desired direction. The nitrides ofTi, V, W, Zr, and Hf, which are orientated toward the (100), (110), or(111) direction, and the oxides of Ir and Ru which are orientated toward(100) may be cited as a preferred example.

The reducing agent is generated on the side of the protection layer(negative electrode) 13 after the set operation, preferably theprotection layer 13 has a function of preventing the recording layer 12from reacting with the atmosphere.

Examples of such material include semiconductor materials such asamorphous carbon, diamond-like carbon, and SnO₂.

The electrode layer 13A may act as a protection layer which protects therecording layer 12, or the protection layer may be provided instead ofthe electrode layer 13A. In such cases, the protection layer may be madeof an insulating material or a conductive material.

In order to efficiently heat the recording layer 12 in the resetoperation, a heater layer (made of a material having resistivity ofabout 10⁻⁵Ω·cm or more) may be provided on the negative electrode side(in the example, on the side of the electrode layer 13A).

(2) A basic principle of recording/erasing/reproducing of information inthe information recording/reproducing device of the second example ofthe invention will be described.

FIG. 2 shows a structure of a recording unit.

In FIG. 2, the reference number 11 denotes an electrode layer, thereference number 12 denotes a recording layer, and the reference number13A denotes an electrode layer (or protection layer).

The recording layer 12 includes a first chemical compound 12A and asecond chemical compound 12B. The first chemical compound 12A isexpressed by A_(x)M_(y)X₄, and the first chemical compound 12A isdisposed on the side of the electrode layer 11. The second chemicalcompound 12B is disposed on the side of the electrode layer 13A, and thesecond chemical compound 12B has a vacant site which the cation elementof the first chemical compound 12A can occupy.

A small white circle indicated by a bold line in the first chemicalcompound 12A expresses a transition element ion (typically M ion), asmall white circle in the first chemical compound 12A expresses adiffusion ion (typically A ion), and a black circle in the secondchemical compound 12B expresses a transition element ion (M2 ion). Alarge circle expresses an anion (X ion in the first chemical compound12A, and X2 ion in the second chemical compound 12B).

As shown in FIG. 3, the first and second chemical compounds 12A and 12Bconstituting the recording unit 12 may be stacked into at least twolayers.

In the recording unit of FIG. 2, potentials are imparted to theelectrode layers 11 and 13A to generate the potential gradient in therecording layer 12 such that the first chemical compound 12A becomes thepositive electrode side while the second chemical compound 12B becomesthe negative electrode side. Therefore, a part of the diffusion ions inthe first chemical compound 12A transfers in the crystal and proceedsinto the second chemical compound 12B on the negative electrode side.

Because the vacant site which the diffusion ion can occupy exists in thecrystal of the second chemical compound 12B, the diffusion ion whichtransfers from the first chemical compound 12A can occupy the vacantsite.

Accordingly, the valence of the transition element ion increases in thefirst chemical compound 12A, and the valence of the transition elemention decreases in the second chemical compound 12B.

For the initial state (reset state), when it is assumed that the firstand second chemical compounds 12A and 12B are in the high-resistancestate (insulating material), a part of the diffusion ions in the firstchemical compound 12A transfers into the second chemical compound 12B.Therefore, conductive carriers are generated in the first and secondchemical compounds 12A and 12B, and the first and second chemicalcompounds 12A and 12B have the electric conductivity.

As described above, the current/voltage pulse is imparted to therecording layer 12 to decrease the electrical resistance of therecording layer 12, thereby realizing the set operation (recording).

At the same time, the electron also transfers from the first chemicalcompound 12A to the second chemical compound 12B. However, because theFermi level of the electron of the second chemical compound 12B ishigher than the Fermi level of the electron of the first chemicalcompound 12A, the total energy of the recording layer 12 rises.

Because the high energy state is continued after the set operation iscompleted, there is a risk of naturally returning the recording layer 12from the set state (low-resistance state) to the reset state(high-resistance state).

However, the use of the recording layer 12 of the second example of theinvention avoids the risk. That is, the set state can be maintained.

This is because the so-called ion transfer resistance acts.

The valence of the diffusion ion in the first chemical compound 12Aplays a role of the action. The divalent diffusion ion has an importantmeaning.

When the diffusion ion has the valence of one like the Li ion, thesufficient ion transfer resistance can not be obtained in the set state,and the diffusion ion is instantaneously returned from the secondchemical compound 12B to the first chemical compound 12A. In otherwords, the sufficiently long retention time can not be obtained.

When the diffusion ion has the valence of at least three, because thevoltage necessary for the set operation is increased, possibly thecollapse of the crystal is caused in the worst case.

Accordingly, preferably the diffusion ion has the valence of two in theinformation recording/reproducing device.

Because the oxidant is generated on the positive electrode side afterthe set operation is completed, preferably the electrode layer 11 ismade of a material (for example, electric conductive oxide) which ishardly oxidized and does not have the ion conductivity in this case. Thepreferred example is described above.

In the reset operation (erasing), the heating of the recording layer 12may be performed to promote the phenomenon in which the diffusion ionwhich can occupy the vacant site of the second chemical compound 12B isreturned to the first chemical compound 12A.

Specifically, the recording layer 12 can be easily returned to theoriginal high-resistance state (insulating material) by utilizing theJoule heat and the residual heat of the Joule heat. The Joule heat isgenerated by supplying the large current pulse to the recording layer12.

Thus, the electrical resistance of the recording layer 12 is increasedby supplying the large current pulse to the recording layer 12, therebyrealizing the reset operation (erasing). Alternatively, the resetoperation may be performed by applying an electric field opposite tothat of the set operation.

In order to realize the low power consumption, it is important to usethe structure having the transfer path, in which the ion radius of thediffusion ion is optimized such that the diffusion ion can transfer inthe crystal without causing the collapse of the crystal.

When the material and crystal structure described in the outline areused as the second chemical compound 12B while coming into contact withthe first chemical compound having the spinel structure in which thecation transfers easily, the condition is satisfied to effectivelyrealize the low power consumption.

When the oxide spinel is used, at least one material selected from thegroup of Zn, Cd, and Hg is preferably used as the diffusion ion element,and at least one material selected from the group of Cr, Mo, W, Mn andRe is preferably used as the transition element. A part of a crystal asthe transition element can replace to one element selected from thegroup of Fe, Co, Ni, Al and Ga.

From the viewpoints of the ease of cation transfer and the degree ofcoincidence of the lattice constant, preferably M2X2₂ having thehollandite structure is used as the second chemical compound which isused while preferably combined with the spinel material, and mostpreferably Ti is used as M2 and O is used as X2.

The spinel structure having many vacant sites is cited as an example ofthe second chemical compound having the high degree of coincidence ofthe lattice constant with that of the first chemical compound. When thematerial having the spinel structure having many vacant sites is used asthe second chemical compound, preferably the second chemical compound issuitably orientated.

The structure of crystal is distorted from a cubic crystal, when x ofZnMn_(2-x)Ms_(x)O₄ (where Ms is a displacement element) is small.Therefore, it is preferable that the coincidence of lattice constantbetween the first and second chemical compounds is improved, when amaterial made of a hollandite structure is used as the second chemicalcompound.

The preferred film thickness range of the second chemical compound willbe described below.

In order to obtain the effect of the vacant site which the diffusion ioncan occupy, preferably the second chemical compound has the filmthickness of 1 nm or more.

On the other hand, when the number of vacant sites in the secondchemical compound is larger than the number of diffusion ions in thefirst chemical compound, the effect of the resistance change of thesecond chemical compound is reduced. Therefore, preferably the number ofvacant sites in the second chemical compound is less than or equal tothe number of diffusion ions in the first chemical compound that existsin the same sectional area.

Because density of the diffusion ion in the first chemical compound issubstantially equal to the density of the vacant site in the secondchemical compound, preferably the film thickness of the second chemicalcompound is substantially less than or equal to the film thickness ofthe first chemical compound.

Generally, a heater layer (made of a material having resistivity ofabout 10⁻⁵Ω·cm or more) may be provided on the negative electrode sidein order to further promote the reset operation.

In the probe memory, because a reducing material is deposited on thenegative electrode side, preferably a surface protection layer isprovided in order to prevent the reducing material from reacting withatmosphere.

The heater layer and the surface protection layer may be made of onematerial having functions of both the heater layer and the surfaceprotection layer. For example, semiconductor materials such as amorphouscarbon, diamond-like carbon, and SnO₂ have both the heater function andthe surface protection function.

The current pulse is passed through the recording layer 12 to detect theresistance of the recording layer 12, thereby easily performing thereproducing.

However, it is necessary to set the current pulse to a minute value ofan extent that the material constituting the recording layer 12 does notcause the resistance change.

3. EMBODIMENTS

Next, explanation will be made on some embodiments considered to be thebest.

Hereinafter, explanation will made about two cases: a first case inwhich the example of the present invention is applied to a probe memoryand a second case in which the example of the present invention isapplied to a semiconductor memory.

(1) Probe Memory

A. Structure

FIGS. 4 and 5 show the probe memory according to the example.

A recording medium is arranged on an XY scanner 14. A probe array isarranged to face the recording medium.

The probe array has a substrate 23 and probes (heads) 24 arranged in anarray shape at one face side of the substrate 23. Each of the probes 24is comprised by, for instance, a cantilever, and driven by multiplexdrivers 25, 26.

Each of the probes 24 can operate individually by using a micro actuatorin the substrate 23. However, here, there will be explained an examplein which access is performed to data areas of the recording medium whilecausing all the probes to operate in the same manner.

Firstly, by using the multiplex drivers 25, 26, all the probes 24 arecaused to perform a reciprocating operation at a constant frequency inthe X direction, to read position information of the Y direction from aservo area of the recording medium. The position information in the Ydirection is transferred to a driver 15.

The driver 15 drives the XY scanner 14 based on the positioninformation, causes the recording medium to move in the Y direction, andperforms positioning of the recording medium and the probe.

After completing the positioning of the both, read or write of data isperformed simultaneously and continuously to all the probes 24 on thedata area.

The read and write of the data are performed continuously because theprobe 24 is performing the reciprocating operation in the X direction.Further, the read and write of the data are executed in every one lineto the data area by sequentially changing the position in the Ydirection of the recording medium.

Meanwhile, it is also possible to read the position information from therecording medium while causing the recording medium to performreciprocating movement at a constant frequency in the X direction, andthen cause the probe 24 to move in the Y direction.

The recording medium is comprised, for instance, a substrate 20, anelectrode layer 21 on the substrate 20, and a recording layer 22 on theelectrode layer 21.

The recording area 22 has data areas, and servo areas arrangedrespectively at both ends in the X direction of the data areas. Dataareas occupy a principal part of the recording layer 22.

Servo burst signals are recorded in the servo area. The servo burstsignals indicate the position information in the Y direction in the dataarea.

In the recording layer 22, in addition to these pieces of information,there are arranged an address area in which address data is recorded anda preamble area to take synchronization.

The data and the servo burst signal are recorded in the recording layer22 as recording bits (the electric resistance change).

“1”, “0” information of the recording bit is read by detecting theelectric resistance of the recording layer 22.

In the present example, one probe (head) corresponding to one data areais provided, and one probe corresponding to one servo area is provided.

The data area is comprised by tracks. The track of the data area isspecified by address signals read from the address area. Further, theservo burst signal read from the servo area is for causing the probe 24to move to the center of the track to eliminate read error of therecording bit.

Here, the X direction is caused to correspond to a down track direction,and the Y direction is caused to correspond to an up track direction,thereby making it possible to utilize the head position controltechnique of HDD.

B. Recording/Reproducing Operation

Explanation will next be made about recording/reproducing operation ofthe probe memory of FIGS. 4 and 5.

FIG. 6 shows a state at the time of recording (set operation).

The recording medium is comprised the electrode layer 21 on thesubstrate (for instance, semiconductor chip) 20, the recording layer 22on the electrode layer 21, and the protection layer 13B on the recordinglayer 22. The protection layer 13B is comprised, for instance, a thininsulating material.

A recording operation is performed by generating the potential gradientsin a recording bit 27 by applying a voltage to a surface of therecording bit 27 of the recording layer 22. Specifically, it is onlynecessary to supply a current/voltage pulse to the recording bit 27.

First Example

The materials of FIG. 1 are used for the recording layer in the firstexample.

Firstly, as shown in FIG. 7, there is prepared a state where theelectric potential of the probe 24 is relatively lower than the electricpotential of the electrode layer 21. The probe 24 may be supplied with anegative electric potential, when the electrode layer 21 has a fixedelectric potential, for instance, ground potential.

The current pulse is generated by emitting electrons toward theelectrode layer 21 from the probe 24 while using, for instance, anelectron generating source or hot electron source. Alternatively, it isalso possible to bring the probe 24 into contact with the surface of therecording bit 27 to apply the voltage pulse.

At this time, for instance, in the recording bit 27 of the recordinglayer 22, part of diffusion ions moves to the probe (cathode) 24 side,and the number of cations inside the crystal relatively decreases incomparison to the number of anions. Further, diffusion ions moved to theprobe 24 side separate out as the metal, while receiving electrons fromthe probe 24.

In the recording bit 27, the anions become excessive. As a result, avalence of diffusion ions in the recording bit 27 increases. That is,the recording bit 27 comes to have electron conductivity due toimplantation of carrier by phase change, thereby decreasing theresistance in the thickness direction, and then the recording (setoperation) is completed.

Similarly, the current pulse for recording can also be generated bypreparing the state where the electric potential of the probe 24 isrelatively higher than the electric potential of the electrode layer 21.

FIG. 8 shows the reproducing.

The reproducing is performed by causing the current pulse to flowthrough the recording bit 27 of the recording layer 22, followed bydetecting the resistance value of the recording bit 27. However, thecurrent pulse is set to a minute value to the degree that the materialconstituting the recording bit 27 of the recording layer 22 does notcause the resistance change.

For instance, a read current (current pulse) generated by a senseamplifier S/A is caused to flow through the recording bit 27 from theprobe 24, and then, the resistance value of the recording bit 27 ismeasured by the sense amplifier S/A.

If the material according to the example of the present invention isused, it is possible to secure a difference of 10³ or more in theresistance value between the set/reset states.

Meanwhile, in the reproducing, continuous reproducing becomes possibleby scanning the recording medium by the probe 24.

The erase (reset) operation is performed by promoting theoxidation-reduction reaction in the recording bit 27 in such a mannerthat the recording bit 27 of the recording layer 22 is subjected tojoule heating based on the large-current pulse. Alternatively, it isalso possible to apply the pulse providing potential of an inversedirection to the potential difference at the time of the set operation.

The erase operation can be performed in every recording bit 27, or canbe performed on recording bits 27 or on a block unit.

Second Example

The materials of FIG. 2 are used for the recording layer in the secondexample.

Firstly, as shown in FIG. 9, there is prepared a state where theelectric potential of the probe 24 is relatively lower than the electricpotential of the electrode layer 21. It is only necessary to supply anegative potential to the probe 24 when the electrode layer 21 has afixed electric potential, for instance, ground potential.

At this time, part of diffusion ions inside the first chemical compound(anode side) 12A of the recording layer 22 can occupy in the vacant siteof the second chemical compound (cathode side) 12B while moving insidethe crystal. With this, the valence of diffusion ions inside the firstchemical compound 12A increases, while the valence of transition elementions inside the second chemical compound 12B decreases. As a result,conductive carriers are generated inside the crystal of the first andsecond chemical compounds 12A, 12B, and then both come to have theelectrical conductivity.

In this manner, the set operation (recording) is completed.

Meanwhile, concerning the recording operation, assuming that theposition relation of the first and second chemical compounds 12A, 12B isreversed, it is also possible to execute the set operation while makingthe electric potential of the probe 24 relatively higher than theelectric potential of the electrode layer 21.

FIG. 10 shows a state at the time of the reproducing.

The reproducing operation is performed by causing the current pulse toflow through the recording bit 27, followed by detecting the resistancevalue of the recording bit 27. However, the current pulse needs to havea minute value to the degree that the material constituting therecording bit 27 does not cause the resistance change.

For instance, the read current (current pulse) generated by the senseamplifier S/A is caused to flow through the recording layer (recordingbit) 22 from the probe 24, and then, the resistance value of therecording bit is measured by the sense amplifier S/A. When adopting thenew materials described already, it is possible to secure a differenceof 10³ or more in the resistance value between the set/reset states.

Meanwhile, the reproducing operation can be performed continuously byscanning the probe 24.

The reset (erase) operation may be performed by facilitating the actionin which diffusion ions return to first chemical compound 12A from thevacant site inside the second chemical compound 12B while utilizing thejoule heat and its residual heat generated by causing the large-currentpulse to flow through the recording layer (recording bit) 22.Alternatively, it may be performed by applying the pulse providing thepotential difference in an inverse direction to the potential differenceat the time of the set operation.

The erase operation can be performed in every recording bit 27, or canbe performed on recording bits 27 or on a block unit.

C. Summary

According to such probe memory, it is possible to realize a higherrecording density and lower power consumption than those of the presenthard disk or flash memory.

(2) Semiconductor Memory

A. Structure

FIG. 11 shows a cross point type semiconductor memory according to anexample.

Word lines WLi−1, WLi, and WLi+1 extend in diffusion direction, and bitlines BLj−1, BLj, and BLj+1 extend in the Y direction.

Each one end of the word lines WLi−1, WLi, and WLi+1 is connected to aword line driver & decoder 31 via a MOS transistor RSW as a selectionswitch, and each one end of the bit lines BLj−1, BLj, and BLj+1 isconnected to a bit line driver & decoder & read circuit 32 via a MOStransistor CSW as a selection switch.

Selection signals Ri−1, Ri, and Ri+1 for selecting one word line (row)are input to a gate of the MOS transistor RSW, and selection signalsCi−1, Ci, and Ci+1 for selecting one bit line (column) are input to agate of the MOS transistor CSW.

A memory cell 33 is arranged at each intersection part of the word linesWLi−1, WLi, and WLi+1 and the bit lines BLj−1, BLj, and BLj+1. Thememory cell 33 has a so called cross point cell array structure.

A diode 34 for preventing a sneak current at the time ofrecording/reproducing is added to the memory cell 33.

FIG. 12 shows a structure of a memory cell array part of thesemiconductor memory of FIG. 11.

The word lines WLi−1, WLi, and WLi+1 and the bit lines BLj−1, BLj, andBLj+1 are arranged on a semiconductor chip 30, and the memory cells 33and the diodes 34 are arranged in the intersection parts of thesewirings.

A feature of such a cross point type cell array structure lies in apoint that, since it is not necessary to connect the MOS transistorindividually to the memory cell 33, it is advantageous for highintegration. For instance, as shown in FIGS. 14 and 15, it is possibleto give the memory cell array a three-dimensional structure, by stackingthe memory cells 33.

For instance, as shown in FIG. 13, the memory cell 33 is comprised astack structure of a recording layer 22, a protection layer 13B and aheater layer 35. One bit data is stored in one memory cell 33. Further,the diode 34 is arranged between the word line WLi and the memory cell33. Buffer layer may be provided between the word line WLi and the diode34. Buffer layer may be provided between the bit line BLj and theprotection layer 13B.

B. Recording/Reproducing Operation

A recording/reproducing operation will be explained using FIGS. 11 to13.

Here, it is assumed that the recording/reproducing operation is executedwhile selecting the memory cell 33 surrounded by dotted line A.

First Example

The first example is a case in which the materials of FIG. 1 are usedfor the recording layer.

Since it is adequate for the recording (set operation) to apply thevoltage to the selected memory cell 33 followed by generating potentialgradients inside the memory cell 33 to cause current pulses to flowtherein, for instance, there is prepared a state where the electricpotential of the word line WLi is relatively lower than the electricpotential of the bit line BLj. It is only necessary to provide anegative potential to the word line WLi when the bit line BLj has thefixed potential, for instance, ground potential.

At this time, in the selected memory cell 33 surrounded by the dottedline A, part of diffusion ions moves to the word line (cathode) WLiside, and cations inside the crystal relatively decrease to anions.Further, diffusion ions having moved to the word line WLi side separateout as metal while receiving the electrons from the word line WLi.

In the selected memory cell 33 surrounded by the dotted line A, anionsbecome excessive, and as a result, the valence of diffusion ions insidethe crystal is caused to increase. That is, the selected memory cell 33surrounded by the dotted line A comes to have larger electricalconductivity due to implantation of carriers caused by phase change,thereby completing the recording (set operation).

Similarly, at the time of recording, with respect to non selected wordlines WLi−1, WLi+1, and non selected bit lines BLj−1, BLj+1, it ispreferable that all are biased into the same electric potential.

Further, at the time of standby before recording, it is preferable forall of the word lines WLi−1, WLi, and WLi+1, and the bit lines BLj−1,BLj, and BLj+1, to become pre-charged.

Further, the current pulse for recording may be generated by preparingthe state where the electric potential of the word line WLi isrelatively higher than the electric potential of the bit line BLj.

The reproducing is performed by detecting a resistance value of thememory cell 33 while causing the current pulse to flow through theselected memory cell 33 surrounded by the dotted line A. However, it isnecessary for the current pulse to be a minute value to the degree thatthe material constituting the memory cell 33 does not cause resistancechanges.

For instance, the read current (current pulse) generated by a readcircuit is caused to flow through the selected memory cell 33 surroundedby the dotted line A from the bit line BLj, and the resistance value ofthe memory cell 33 is measured by the read circuit. If adopting the newmaterials described above, the difference in the resistance valuebetween the set/reset states can be secured at 10³ or more.

The erase (reset) operation is performed by facilitating theoxidation-reduction reaction in the memory cell 33 while performingjoule heating of the selected memory cell 33 surrounded by the dottedline A by a large-current pulse.

Second Example

The second example is a case in which the materials of FIG. 2 are usedfor the recording layer.

Since it is adequate for the recording (set operation) to apply thevoltage to the selected memory cell 33 followed by generating potentialgradients inside the memory cell 33 to cause current pulses to flowtherein, for instance, there is prepared a state where the electricpotential of the word line WLi is relatively lower than the electricpotential of the bit line BLj. It is only necessary to provide anegative potential to the word line WLi when the bit line BLj has thefixed potential, for instance, ground potential.

At this time, in the selected memory cell 33 surrounded by the dottedline A, part of diffusion ions inside the first chemical compound movesto the vacant site of the second chemical compound. For this reason, thevalence of transition element ions inside the first chemical compoundincreases, and the valence of transition element ions inside the secondchemical compound decreases. As a result, the conductive carriers aregenerated inside the crystal of the first and second chemical compounds,and both come to have electrical conductivity.

Herewith, the set operation (recording) is completed.

Likewise, at the time of recording, with respect to non selected wordlines WLi−1, WLi+1, and non selected bit lines BLj−1, BLj+1, it ispreferable that all are biased with the same electric potential.

Further, at the time of standby before recording, it is preferable forall of the word lines WLi−1, WLi, and WLi+1, and the bit lines BLj−1,BLj, and BLj+1, to become pre-charged.

Further, the current pulse may be generated by preparing the state wherethe electric potential of the word line WLi is relatively higher thanthe electric potential of the bit line BLj.

The reproducing operation is performed by detecting the resistance valueof the memory cell 33 while causing the current pulse to flow throughthe selected memory cell 33 surrounded by the dotted line A. However, itis necessary for the current pulse to be a minute value to the degreethat the material constituting the memory cell 33 does not causeresistance changes.

For instance, the read current (current pulse) generated by the readcircuit is caused to flow through the selected memory cell 33 surroundedby the dotted line A from the bit line BLj, and the resistance value ofthe memory cell 33 is measured by the read circuit. If adopting the newmaterials described above, the difference in the resistance valuebetween the set/reset states can be secured at 10³ or more.

The reset (erase) operation may be performed by facilitating the actionin which diffusion ion element returns to the first chemical compoundfrom the vacant site inside the second chemical compound while utilizingthe joule heat and its residual heat generated by causing thelarge-current pulse to flow through the selected memory cell 33surrounded by the dotted line A.

Here, when the inside of the recording layer 22 formed at theintersection part of the word line WLi and the bit line BLj exists in apolycrystalline state or a monocrystalline state, it is preferable sincediffusion of the ions inside the crystal easily occurs. However, also inthis case, when the size of the crystal grain differs largely atrespective memory cells, there is a possibility that the characteristicof the recording layer in respective memory cells varies. Therefore, itis preferable that in the respective memory cell, the size of crystalgrain is approximately uniform, and that the distribution thereoffollows the distribution having a single peak. In this case, it isassumed that the size of the crystal grain severed at an interface ofeach intersection part is not taken into consideration at the timedistribution is obtained. In order to utilize movement of the diffusionions inside the crystal structure, it is preferable that the size of thecrystal grains in the recording film cross sectional direction is 3 nmor more, more preferably 5 nm or more. Assuming that the size of theintersection part becomes smaller than about 20 nm, it is preferablethat the number of the crystal grains included in the respectiveintersection parts is 10 or less. Further, it is more preferable thatthe number of the crystal grains is 4 or less.

Next, there is considered the size of the crystal grain in the filmthickness direction. In order that the resistance change is generatedefficiently by the diffusion inside the crystal structure, it ispreferable for the size in the film thickness direction of the crystalgrain to be of the same degree or more as the film thickness. However,when layering no second chemical compound, the recording layer may havea minimal amorphous part at an upper part or lower part of the crystalpart of the first chemical compound. This will be explained using FIGS.30 and 31. As described using FIG. 1, ions separate out as A metalinside the recording layer, after being diffused via the diffusion path.At this time, when A ions separate out at an interface part of the firstchemical compound being in the amorphous state while diffusing to an endpart of crystal grain of the first chemical compound, it is preferablebecause there is the vacancy to be occupied by A ions. However, when thefilm thickness t1 of the layer being in the amorphous state becomesexcessively large, the recording layer as a whole does not cause theresistance change efficiently. Generally, the resistance of theamorphous part takes a value between a resistance of the first chemicalcompound having an insulating state and a resistance of the firstchemical compound having a conductive state. Since the resistance changeof the amorphous layer due to movement of A ions is not large, in orderthat the resistance change of the recording film is made more than anorder of magnitude, it is preferable for the film thickness t1 of theamorphous layer to be 1/10 or less of t2.

The amorphous layer may exist on either the upper part or lower part ofthe first chemical compound. However, in order to orient the firstchemical compound in a required direction, generally, orientationcontrol is performed by using a lower layer which agrees with the firstchemical compound in lattice constant, and therefore, it is preferablefor the amorphous part to exist on the upper part of the first chemicalcompound.

Further, the amorphous layer may be generated at the time a next layercontacting the recording layer is formed. In such a case, thecomposition of the amorphous layer, which is different from thecomposition inside the first chemical compound, includes part of thematerials of the next layer contacting the recording layer, and theamorphous layer has an effect of enhancing the adhesion property betweenthe recording film material and the next layer. In this case, filmthickness t1 of the amorphous layer becomes 10 nm or less. It is morepreferable for t1 to be 3 nm or less.

Continuously, there is considered the interface of the respectiveinterconnection parts. When the recording layer is subjected to aprocess in which the recording layer is fabricated in the same shape asthe word line after forming the recording layer uniformly, there is apossibility that the characteristic of the fabricated face of therecording layer is different from that inside the crystal. As a methodfor avoiding this influence, there is a method in which a uniformrecording layer is used without processing, by using the recording layerto become an insulator at the time of film formation. As shown in FIG.28, it is only necessary that the recording layer is formed on the wordlines and the insulator, when a space between the word lines is embeddedwith materials having an insulating property. Alternatively, as shown inFIG. 29, the recording layer may be formed on the word line and on thesubstrate, when the recording film material functions as an insulator ofthe space between the word lines. Thus, it is possible to form arbitraryfilms before forming the recording layer. In FIGS. 28 and 29, there isshown an example in which a buffer layer is formed to suppress diffusionof the recording layer material before the recording layer is formed.The buffer layer may be provided all over the lower part of therecording layer material in advance, when the buffer layer is made ofthe insulator. In FIGS. 28 and 29, the recording film is uniform isshown. However, similarly, it is possible to alleviate the influence ofa processed face, when the recording layer is processed only in thedirection of the bit line or the word line, or when the recording layeris processed to be larger than the respective intersection points.

C. Summary

According to such semiconductor memory, a higher recording density andlower power consumption than those of the existing hard disk or flashmemory can be realized.

(3) Others

Although the probe memory and the semiconductor memory are described inthe examples, the materials and principles proposed in the examples ofthe invention can also be applied to a recording medium such as acurrent hard disk.

4. METHOD FOR MANUFACTURING THE RECORDING MEDIUM

A method for manufacturing the recording medium of the example of theinvention will be described below.

(1) The structure of the recording medium of FIG. 6 is described by wayof example.

It is assumed that the substrate 20 is a glass disk having the diameterof about 60 mm and the thickness of about 1 mm. Platinum (Pt) isevaporated on the substrate 20 to form the electrode layer 21 having thethickness of about 500 nm.

Using a target in which a composition is adjusted such that ZnV₂O₄ isdeposited, RF magnetron sputtering is performed at a temperature of 300to 600° C. in the atmosphere of argon (Ar) 95% and oxygen (O₂) 5% toform ZnV₂O₄ having the thickness of about 10 nm on the electrode layer21. ZnV₂O₄ comprises a part of the recording layer 22.

TiO₂ having the thickness of about 3 nm is formed on ZnV₂O₄ by the RFmagnetron sputtering. As a result, the recording layer 22 has thelaminated structure of ZnV₂O₄ and TiO₂.

Finally the protection layer 13B is formed on the recording layer 22 tocomplete the recording medium of FIG. 6.

(2) The structure of the recording medium of FIG. 12 is described by wayof example.

After the Si substrate protected by the thermally-oxidized film isplanarized, for example, by CMP, a layer made of a conductive materialis deposited on the Si substrate. A metallic material such as W, Ta, Al,or Cu, or an alloy, metal silicide, or nitride such as TiN and WC, orcarbide is used as the conductive material. A highly-doped silicon layermay also be used as the conductive material.

A semiconductor layer made of Si, Ge, GaAs or the like is provided inorder to form a diode. Typically the semiconductor layer includes apolysilicon layer of Si. Alternatively the semiconductor layer mayinclude an amorphous layer. Typically, after a highly-dopedsemiconductor layer (for example, p-type) is provided, a semiconductorlayer in which a dopant having the reverse property (for example,n-type) is doped at a low level is provided to form a diode layer.

Then, as necessary, a highly-doped semiconductor layer having thereversed property (for example, n-type) may be provided in order todecrease an interfacial resistance and a metal silicide layer may beprovided in order to reset a crystalline influence of the diode layer.Alternatively, a buffer layer may be provided in order to control theorientation of the recording layer as necessary.

The case in which a TiN layer is provided as the buffer layer will bedescribed by way of example. It is well known that TiN orientated towardthe (100) direction is easily obtained and the TiN layer having thelarge crystal grain of about 20 nm is easily obtained when the TiN layeris deposited with low power. Therefore, in the manufacturing method ofthe example, the TiN layer deposited with low power is inserted as thebuffer layer.

Then the recording layer (ZnV₂O₄ layer) having the spinel structure isdeposited. As described above, using the target in which the compositionis adjusted such that ZnV₂O₄ is deposited, the RF magnetron sputteringis performed at the temperature of 300 to 600° C. in the atmosphere ofargon (Ar) 95% and oxygen (O₂) 5% to form ZnV₂O₄ having the thickness ofabout 10 nm. When the buffer layer having the large crystal grain isprovided while a ratio of the lattice constant of the buffer layer andthe lattice constant of the first chemical compound is close to aninteger, the orientated first chemical compound having the large crystalgrain is easy to obtain. Because the lattice constant ratio of the(100)-orientated TiN and the (100)-orientated spinel structuresubstantially becomes an integer, the (100)-orientated spinel structureis easily obtained on the (100)-orientated TiN buffer layer. Generallyit is well known that the crystal grain size of the buffer layer issubstantially equal to the crystal grain size of the first chemicalcompound.

For example, SnO₂ is deposited as the protection layer.

A mask process and a first etching process are performed subsequent tothe film deposition processes to obtain sheet-like structures arrayed inparallel.

The space of the sheet-like structure is filled with the insulatingmaterial, and the conductive layer which comprises the upper electrodeis deposited after a planarization process. A mask process and a secondetching process are performed to process the sheet-like structures in adirection orthogonal to the first etching process. Then, the spaceportion is filled with the insulating material, and the planarizationprocess is performed, which allows the memory of FIG. 12 to be obtained.

As described above with reference to the manufacturing method, when thefirst chemical compound which comprises the recording layer is depositedon the underlying layer having the large crystal grain, preferably thefirst chemical compound layer having the large grain size is easilyobtained. At this point, preferably the underlying layer is made of amaterial having good adhesion to the first chemical compound layer. Whenthe lattice constant ratio of the underlying layer and the recordinglayer comes close to an integer, preferably not only the orientation ofthe recording layer can be controlled, but also the large crystal grainof the recording layer can be retained.

5. EXPERIMENTAL EXAMPLE

Experimental examples in which some samples are prepared to evaluate aresistance ratio of the reset state (erasing state) and the set state(writing state) will be described.

The recording medium having the structure of FIG. 6 is used as thesample.

A probe pair in which a diameter of a leading end is steepled to 10 nmor less is used in the evaluation.

One of the probes is brought into contact with the protection layer 13Band grounded. The other probe is brought into contact with the lowerelectrode layer 21 to perform the writing/erasing. For example, thewriting is performed by applying the pulse having the voltage of 1 V andthe width of 10 ns to the recording layer 22. For example, the erasingis performed by applying the pulse having the voltage of 0.2 V and thewidth of 100 ns to the recording layer 22.

For example, the reading is performed using the probe pair between thewriting and the erasing. In the reading, the pulse having the voltage of0.1 V and the width of 10 ns is applied to the recording layer 22 tomeasure the resistance of the recording layer (recording bit) 22.

(1) First Experimental Example

The sample of the first experimental example is prepared as follows.

The electrode layer 21 includes the Pt film which is formed on the disk,and the Pt film has the thickness of about 500 nm. The recording layer22 is made of ZnCr₂O₄, and the protection layer 13B is made ofdiamond-like carbon (DLC).

For example, ZnCr₂O₄ having the thickness of about 10 nm is formed onthe disk by performing the RF magnetron sputtering in the atmosphere of95% of Ar and 5% of O₂ while the disk is maintained at the temperaturerange of 300 to 600° C. The diamond-like carbon having the thickness ofabout 3 nm is formed on ZnCr₂O₄ by, for example, the CVD method.

The resistance after the writing becomes of the order of 10³Ω, and theresistance after the erasing becomes of the order of 10⁷Ω. Theresistance ratio of both is about 10⁴, and it is confirmed that asufficient margin can be secured in the reading.

(2) Second Experimental Example

In the second experimental example, the same sample as the firstexperimental example is used except that the recording layer is made ofZnMn₂O₄.

The resistances after the writing/erasing become of the order of10³Ω/10⁷Ω. The resistance ratio of both is about 10⁴, and it isconfirmed that the sufficient margin can be secured in the reading.

(3) Third Experimental Example

In the third experimental example, the same sample as the firstexperimental example is used except that the recording layer is formedby the laminated structure of Zn_(0.5)Mn₂O₄ and Zn. Zn_(0.5)Mn₂O₄ isformed by the sputtering method and Zn is formed with the thickness ofabout 10 nm.

While the resistance in the initial state is of the order of 10⁸Ω, theresistance after the writing becomes of the order of 10³Ω, and theresistance after the erasing becomes of the order of 10⁷Ω. Theresistance ratio of the writing/erasing is 10⁴ to 10⁵Ω, and it isconfirmed that the sufficient margin can be secured in the reading.

(4) Fourth Experimental Example

In the fourth experimental example, the same sample as the firstexperimental example is used except that the recording layer is made ofZn_(1.2)Cr_(1.8)O₄ and the protection layer is made of SnO₂.

While the resistance in the initial state is of the order of 10⁶Ω, theresistance after the writing becomes of the order of 10²Ω, and theresistance after the erasing becomes of the order of 10⁶Ω. Theresistance ratio of the writing/erasing is about 10⁴, and it isconfirmed that the sufficient margin can be secured in the reading.

Then the evaluation is performed by pulse erasing. The probe pair inwhich a diameter of the leading end is steepled to 10 nm or less is usedin the evaluation.

One of the probes is brought into contact with the protection layer 13Band grounded. The other probe is brought into contact with the lowerelectrode layer 21 to perform the writing/erasing. For example, thewriting is performed by applying the pulse having the voltage of 3 V andthe width of 10 ns to the recording layer 22. For example, the erasingis performed by applying the pulse having the voltage of −3 V and thewidth of 10 ns to the recording layer 22.

The reading is performed using the probe pair between the writing andthe erasing. In the reading, the pulse having the voltage of 0.1 V andthe width of 10 ns is applied to the recording layer 22 to measure theresistance of the recording layer (recording bit) 22.

(5) Fifth Experimental Example

The recording medium having the structure of FIG. 6 is used as thesample of the fifth experimental example. In the fifth experimentalexample, the same sample as the first experimental example is usedexcept that the recording layer is made of ZnMn₂O₄ and the protectionlayer is made of SnO₂.

The resistances after the writing/erasing become of the order of10³Ω/10⁷Ω. The resistance ratio of both is about 10⁴, and it isconfirmed that the sufficient margin can be secured in the reading.

(6) Sixth Experimental Example

In the sixth experimental example, the same sample as the fifthexperimental example is used except that the recording layer is made ofZnCr_(1.7)Al_(0.3)O₄.

The resistances after the writing/erasing become of the order of10⁴Ω/10⁸Ω. The resistance ratio of both is about 10⁴, and it isconfirmed that the sufficient margin can be secured in the reading.

(7) Seventh Experimental Example

In the seventh experimental example, the same sample as the fifthexperimental example is used except that the recording layer is made ofZnMn_(1.8)Al_(0.2)O₄.

The resistances after the writing/erasing become of the order of10⁴Ω/10⁸Ω. The resistance ratio of both is about 10⁴, and it isconfirmed that the sufficient margin can be secured in the reading.

(8) Eighth Experimental Example

In the eighth experimental example, the same sample as the fifthexperimental example is used except that the recording layer is made ofZn_(1.1)Mn_(1.9)O₄.

The resistances after the writing/erasing become of the order of10⁴Ω/10⁷Ω. The resistance ratio of both is about 10³, and it isconfirmed that the sufficient margin can be secured in the reading.

(9) Ninth Experimental Example

The recording medium having the structure of FIG. 6 is used as thesample of the ninth experimental example. In the ninth experimentalexample, the recording layer 22 is formed by the laminated structure ofthe first chemical compound and the second chemical compound. The firstchemical compound having the thickness of 10 nm is made of ZnMn₂O₄, andthe second chemical compound having the thickness of 3 nm is made ofTiO₂. The protection layer is made of SnO₂.

The resistances after the writing/erasing become of the order of10⁴Ω/10⁹Ω. The resistance ratio of both is about 10⁵, and it isconfirmed that the sufficient margin can be secured in the reading.

(10) Tenth Experimental Example

In the tenth experimental example, the (111)-orientated TiN is used asthe electrode layer 21, and ZnMn₂O₄ is used as the first chemicalcompound.

Using the Ti target, TiN is deposited on the Si(100) substrate at roomtemperature in the atmosphere of 92.5% gaseous argon and 7.5% gaseous N₂by the RF magnetron sputtering. The (111)-orientated TiN is obtainedwith the film thickness of 50 nm.

Then the first chemical compound of ZnMn₂O₄ is deposited as therecording layer 22 with the thickness of 10 nm. At this point, ZnMn₂O₄is mainly orientated toward the (110) direction.

Finally SnO₂ is deposited as the protection film 13B with the thicknessof 2 nm to obtain the recording medium having the structure of FIG. 6.

The resistances after the writing/erasing become of the order of10²Ω/10⁷Ω. The resistance ratio of both is about 10⁵, and it isconfirmed that the sufficient margin can be secured in the reading.

(11) Comparative Example

In the comparative example, the same sample as the fifth experimentalexample is used except that the recording layer is made of MgO.

In the comparative example, the writing/erasing cannot be performed whenthe pulse having the voltage of 1 V and the width of 10 ns is applied tothe recording layer 22 like the first experimental example. Therefore,the pulse having the voltage of 20 V and the width of 10 ns is appliedto perform the writing, and the pulse having the voltage of −3 V and thewidth of 1 μs is applied to perform the erasing. The resistances afterthe writing/erasing become of the order of 10⁵Ω/10¹³Ω.

As described above, when MgO having the NaCl structure is used as therecording layer, disadvantageously the large voltage is necessary toperform the writing/erasing because the cation hardly diffuses.

(12) Summary

As described above, in all of the samples of the first to tenthexperimental examples, the basic operations of the writing, the erasing,and the reading can be performed.

6. APPLICATION TO A FLASH MEMORY

(1) Structure

The example of the present invention can also be applied to the flashmemory.

FIG. 16 shows a memory cell of the flash memory.

The memory cell of the flash memory is comprised a MIS(metal-insulator-semiconductor) transistor.

A diffusion layer 42 is formed in a surface region of a semiconductorsubstrate 41. A gate insulating layer 43 is formed on a channel regionbetween the diffusion layers 42. A recording layer (ReRAM: ResistiveRAM) 44 according to an example of the present invention is formed onthe gate insulating layer 43. A control gate electrode 45 is formed onthe recording layer 44.

The semiconductor substrate 41 may be a well region, and thesemiconductor substrate 41 and the diffusion layer 42 have reverseconductivity types mutually. The control gate electrode 45 becomes theword line, and is comprised a conductive polysilicon.

The recording layer 44 is comprised the materials shown in FIG. 1, 2 or3.

(2) Fundamental Operation

Explanation will now be made about the fundamental operation using FIG.16.

A set (write) operation is executed by providing an electric potentialV1 to the control gate electrode 45, and providing an electric potentialV2 to the semiconductor substrate 41.

The difference between the electric potentials V1, V2 needs to besufficiently large for the recording layer 44 to cause a phase change ora resistance change, but its direction is not limited particularly.

That is, either V1>V2 or V1<V2 may be applied.

For instance, in an initial state (reset state), assuming that therecording layer 44 is an insulator (resistance is large), the gateinsulating layer 43 becomes quite thick. As a result, a threshold of thememory cell (MIS transistor) becomes high.

When the recording layer 44 is caused to change into a conductor(resistance is small) while providing the electric potentials V1, V2from this state, the gate insulating layer 43 becomes quite thin. As aresult, a threshold of the memory cell (MIS transistor) becomes low.

Note that, although the electric potential V2 is supplied to thesemiconductor substrate 41, the electric potential V2 may be insteadtransferred to the channel region of the memory cell from the diffusionlayer 42.

The reset (erase) operation is executed in such a manner that theelectric potential V1′ is supplied to the control gate electrode 45, theelectric potential V3 is supplied to one of the diffusion layers 42, andthe electric potential V4 (<V3) is supplied to the other one of thediffusion layers 42.

The electric potential V1′ is set to a value exceeding the threshold ofthe memory cell being in the set state.

At this time, the memory cell becomes ON, the electrons flow toward onedirection from the other direction of the diffusion layer 42, and hotelectrons are generated. Since the hot electrons are implanted into therecording layer 44 via the gate insulating layer 43, the temperature ofthe recording layer 44 increases.

Herewith, since the recording layer 44 changes to the insulator(resistance is large) from the conductor (resistance is small), the gateinsulating layer 43 becomes quite thick. Accordingly, the threshold ofthe memory cell (MIS transistor) becomes high.

In this manner, by a similar principle to the flash memory, thethreshold of the memory cell can be changed, and therefore, it ispossible to put the information recording/reproducing device accordingto the example of the present invention into practical use, whileutilizing the technique of the flash memory.

(3) NAND Type Flash Memory

FIG. 17 shows a circuit diagram of a NAND cell unit. FIG. 18 shows astructure of the NAND cell unit according to the example.

An N-type well region 41 b and a P-type well region 41 c are formedinside a P-type semiconductor substrate 41 a. A NAND cell unit accordingto the example of the present invention is formed inside the P-type wellregion 41 c.

The NAND cell unit is comprised of a NAND string comprised memory cellsMC connected in series, and a total of two select gate transistors STconnected one by one to the both ends of the NAND string.

The memory cell MC and the select gate transistor ST have the samestructure. Specifically, these are comprised an N-type diffusion layer42, a gate insulating layer 43 on the channel region between the N-typediffusion layers 42, a recording layer (ReRAM) 44 on the gate insulatinglayer 43, and a control gate electrode 45 on the recording layer 44.

States (insulator/conductor) of the recording layer 44 of the memorycell MC can be changed by the above-described fundamental operation. Onthe other hand, the recording layer 44 of the select gate transistor STis fixed to the set state, that is, the conductor (resistance is small).

One of the select gate transistors ST is connected to a source line SL,and the other one is connected to a bit line BL.

Before set (write) operation, it is assumed that all memory cells insidethe NAND cell unit are in the reset state (resistance is large).

The set (write) operations are performed one by one in order toward thememory cell at the bit line BL side from the memory cell MC at thesource line SL side.

V1 (plus potential) is supplied as the write potential to the selectedword line (control gate electrode) WL, and V_(pass) is supplied as atransfer potential (electric potential by which memory cell MC becomesON) to the non selected word line WL.

Program data is transferred to the channel region of the selected memorycell MC from the bit line BL, in the state that the select gatetransistor ST at the source line SL side is made OFF, and the selectgate transistor ST at the bit line BL side is made ON.

For instance, when the program data is “1”, a write inhibit potential(for instance, electric potential being the same degree as V1) istransferred to the channel region of the selected memory cell MC, sothat the resistance value of the recording layer 44 of the selectedmemory cell MC does not change into the low state from the high state.

Further, when the program data is “0”, V2 (<V1) is transferred to thechannel region of the selected memory cell MC, and the resistance valueof the recording layer 44 of the selected memory cell MC is changed intothe low state from the high state.

In the reset (erase) operation, for instance, V1′ is supplied to all theword lines (control gate electrode) WL to make all the memory cells MCinside the NAND cell unit ON. Further, the two select gate transistorsST are turned ON, V3 is supplied to the bit line BL, and V4 (<V3) issupplied to the source line SL.

At this time, since the hot electrons are implanted to the recordinglayer 44 of all the memory cells MC inside the NAND cell unit, the resetoperation is collectively executed to all memory cells MC inside theNAND cell unit.

The read operation is performed in such a manner that a read potential(plus potential) is supplied to the selected word line (control gateelectrode) WL, and electric potentials by which the memory cell MCbecomes inevitably ON regardless of the data “0”, “1” are supplied tothe non selected word line (control gate electrode) WL.

Further, the two select gate transistors ST are turned ON, and the readcurrent is supplied to the NAND string.

Since the selected memory cell MC, when applied with the read potential,becomes ON or OFF in accordance with data value stored therein, it ispossible to read the data by, for instance, detecting changes of theread current.

In the structure of FIG. 18, the select gate transistor ST has the samestructure as the memory cell MC. However, for instance, as shown in FIG.19, the select gate transistor ST may be a normal MIS transistor withoutforming the recording layer.

FIG. 20 shows a modified example of the NAND type flash memory.

The modified example is characterized in that the gate insulating layerof memory cells MC constituting the NAND string is replaced with aP-type semiconductor layer 47.

When high integration is advanced and the memory cell MC isminiaturized, in a state where the voltage is not supplied, the P-typesemiconductor layer 47 is filled with a depletion layer.

At the time of set (write), a plus write potential (for instance, 3.5V)is supplied to the control gate electrode 45 of the selected memory cellMC, and a plus transfer potential (for instance, 1V) is supplied to thecontrol gate electrode 45 of the non selected memory cell MC.

At this time, a surface of the P-type well region 41 c of memory cellsMC inside the NAND string inverts from P-type to N-type, so that achannel is formed.

Consequently, as described above, when the select gate transistor ST atthe bit line BL side is turned ON, and the program data “0” istransferred to the channel region of the selected memory cell MC fromthe bit line BL, it is possible to perform the set operation.

The reset (erase) can be collectively performed to all the memory cellsMC constituting the NAND string, when, for instance, minus erasepotential (for instance, −3.5V) is supplied to all the control gateelectrodes 45, and the ground potential (0V) is supplied to the P-typewell region 41 c and the P-type semiconductor layer 47.

At the time of the read, the plus read potential (for instance, 0.5V) issupplied to the control gate electrode 45 of the selected memory cellMC, and the transfer potential (for instance, 1V) by which the memorycell MC becomes inevitably ON regardless of the data “0”, “1” issupplied to the control gate electrode 45 of the non selected memorycell MC.

It is assumed that the threshold voltage Vth “1” of the memory cell MCof “1” state should fall in the range of 0V<Vth “1”<0.5V, and thethreshold voltage Vth “0” of the memory cell MC of “0” state should fallin the range of 0.5V<Vth “0”<1V.

Further, the read current is supplied to the NAND string while makingthe two select gate transistors ST ON.

When such state is realized, since current quantity flowing through theNAND string is changed in accordance with the data value stored in theselected memory cell MC, it is possible to read the data by detectingthis change.

Meanwhile, in this modified example, it is desirable that the hole dopeamount of the P-type semiconductor layer 47 is more than that of theP-type well region 41 c, and the Fermi level of the P-type semiconductorlayer 47 is deeper than that of the P-type well region 41 c by about0.5V.

This is because when a plus potential is supplied to the control gateelectrode 45, an inversion from P-type to N-type commences from asurface part of the P-type well region 41 c between the N-type diffusionlayers 42, so that the channel is to be formed.

Accordingly, for instance, at the time of the write, the channel of thenon selected memory cell MC is formed only at an interface between theP-type well region 41 c and the P-type semiconductor layer 47, and atthe time of the read, the channel of memory cells MC inside the NANDstring is formed only at an interface between the P-type well region 41c and the P-type semiconductor layer 47.

That is, even though the recording layer 44 of the memory cell. MC is inthe conductor (set state), the diffusion layer 42 and the control gateelectrode 45 do not short-circuit.

(4) NOR Type Flash Memory

FIG. 21 shows a circuit diagram of a NOR cell unit. FIG. 22 shows astructure of the NOR cell unit according to an example of the presentinvention.

An N-type well region 41 b and a P-type well region 41 c are formedinside a P-type semiconductor substrate 41 a. The NOR cell according tothe example of the present invention is formed inside the P-type wellregion 41 c.

The NOR cell is comprised one memory cell (MIS transistor) MC connectedbetween the bit line BL and the source line SL.

The memory cell MC is comprised an N-type diffusion layer 42, a gateinsulating layer 43 on the channel region between the N-type diffusionlayers 42, a recording layer (ReRAM) 44 on the gate insulating layer 43,and a control gate electrode 45 on the recording layer 44.

The state (insulator/conductor) of the recording layer 44 of the memorycell MC can be changed by the above-described fundamental operation.

(5) 2-Transistor Type Flash Memory

FIG. 23 shows a circuit diagram of a 2-transistor cell unit. FIG. 24shows a structure of the 2-transistor cell unit according to theexample.

The 2-transistor cell unit has been developed recently as a new cellstructure having characteristic of the NAND cell unit in conjunctionwith characteristic of the NOR cell.

An N-type well region 41 b and a P-type well region 41 c are formedinside a P-type semiconductor substrate 41 a. The 2-transistor cell unitaccording to the example of the present invention is formed inside theP-type well region 41 c.

The 2-transistor cell unit is comprised one memory cell MC and oneselect gate transistor ST connected in series.

The memory cell MC and the select gate transistor ST have the samestructure. Specifically, these are comprised an N-type diffusion layer42, a gate insulating layer 43 on the channel region between the N-typediffusion layers 42, a recording layer (ReRAM) 44 on the gate insulatinglayer 43, and a control gate electrode 45 on the recording layer 44.

The state (insulator/conductor) of the recording layer 44 of the memorycell MC can be changed by the above-described fundamental operation. Onthe other hand, the recording layer 44 of the select gate transistor STis fixed to the set state, that is, the conductor (resistance is small).

The select gate transistor ST is connected to the source line SL, andthe memory cell MC is connected to the bit line BL.

States (insulator/conductor) of the recording layer 44 of the memorycell MC can be changed by the above-described fundamental operation.

In the structure of FIG. 24, the select gate transistor ST has the samestructure as the memory cell MC. However, for instance, as shown in FIG.25, the select gate transistor ST may be a normal MIS transistor withoutforming the recording layer.

7. OTHERS

According to the examples of the invention, using the conductivitychange caused by the change of the valence of the transition elemention, the recording (writing) is performed only in the region (recordingunit) to which the electric field is applied. Therefore, the data can berecorded in the extremely small region with the extremely small powerconsumption.

The erasing is performed by applying the heat. When the materialproposed in the examples of the invention is used, the change in oxidestructure is hardly caused. Therefore, the erasing can be performed withthe small power consumption.

Alternatively the erasing can also be performed by applying the electricfield opposite to that of the recording. When the material proposed inthe examples of the invention is used, the erasing is performed withoutdissipating the thermal energy. Therefore, the erasing can be performedwith the extremely small power consumption.

According to the examples of the invention, because the conductiveportion is formed in the insulating material after the writing, thecurrent is concentrated on the conductive portion in the reading.Therefore, the recording principle having the extremely high sensingefficiency can be realized.

According to the examples of the invention, the recording and theerasing can stably be repeated by the combination of theeasily-transferable cation and the transition element ion which stablyretains the matrix structure.

According to the examples of the invention, despite the extremely simplemechanism, the data can be recorded with recording density which cannotbe realized by the conventional technology. Consequently, the examplesof the invention have the large industrial merit as the next-generationtechnology which breaks down the wall of the recording density of thecurrent nonvolatile memory.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An information recording/reproducing device comprising: a recordinglayer; and a recording circuit which records data to the recording layerby generating a phase change in the recording layer, wherein therecording layer includes a first chemical compound having a spinelstructure, and the recording layer is A_(x)M_(y)X₄ (0.1≦x≦2.2,1.0≦y≦2.0) where A includes one selected from a group of Zn, Cd and Hg,M includes one selected from a group of Cr, Mo, W, Mn and Re, and Xincludes O.
 2. The device according to claim 1, wherein M includes oneselected from a group of Fe, Co, Ni, Al and Ga.
 3. The device accordingto claim 1, wherein a C-axis of the recording layer is directed in 45°or less to a surface of the recording layer.
 4. The device according toclaim 1, further comprising a second chemical compound which is adjacentto the first chemical compound, and having a vacant site which cationsin the first chemical compound can occupy.
 5. The device according toclaim 4, wherein the second chemical compound is one of:

_(x)M2X2₂ where

is a vacant site which the cations can occupy, M2 is one selected fromTi, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, and Rh, X2 isone selected from O, S, Se, N, Cl, Br, and I, and 0.3≦x≦1;

_(x)M2X2₃ where

is a vacant site which the cations can occupy, M2 is one selected fromTi, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, and Rh, X2 isone selected from O, S, Se, N, Cl, Br, and I, and 1≦x≦2;

_(x)M2X2₄ where

is a vacant site which the cations can occupy, M2 is one selected fromTi, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, and Rh, X2 isone selected from O, S, Se, N, Cl, Br, and I, and 1≦x≦2;

_(x)M2PO_(z) where

is a vacant site which the cations can occupy, M2 is one selected fromTi, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, and Rh, P is aphosphorus, O is an oxygen, 0.3≦x≦3, and 4≦z≦6; and

_(x)M2O₅ where

is a vacant site which the cations can occupy, M2 is one selected fromTi, Ge, Sn, V, Cr, Mn, Fe, Co, Ni, Nb, Ta, Mo, W, Re, Ru, and Rh, O isan oxygen, and 0.3≦x≦2.
 6. The device according to claim 4, wherein thesecond chemical compound has one of a hollandite structure, ramsdellitestructure, anatase structure, brookite structure, pyrolusite structure,ReO₃ structure, MoO_(1.5)PO₄ structure, TiO_(0.5)PO₄ structure, FePO₄structure, βMnO₂ structure, γMnO₂ structure, and λMnO₂ structure.
 7. Thedevice according to claim 4, wherein a Fermi level of electrons of thefirst chemical compound is lower than a Fermi level of electrons of thesecond chemical compound.
 8. The device according to claim 1, whereinthe recording circuit includes a probe to locally apply the voltage to arecording unit of the recording layer.
 9. The device according to claim1, wherein the recording circuit includes a word line and a bit linesandwiching the recording layer.
 10. The device according to claim 9,wherein the recording layer has 10 or less crystal grains in a crosssection of the word line and the bit line.
 11. The device according toclaim 9, wherein the recording layer is a monocrystal in a cross sectionof the word line and the bit line.


12. The device according to claim 9, wherein the recording circuit isconnected between the word line and the bit line.
 13. The deviceaccording to claim 1, wherein a size of a crystal grain of the recordingcircuit is 3 nm or more.
 14. The device according to claim 1, whereinthe recording circuit includes a MIS transistor having a gate electrodeand a gate insulating layer, and the recording layer is disposed betweenthe gate electrode and the gate insulating layer.
 15. The deviceaccording to claim 1, wherein the recording circuit includes twodiffusion layers in a semiconductor substrate, a semiconductor layer onthe semiconductor substrate between the two diffusion layers, and a gateelectrode above the semiconductor layer, wherein the recording layer isdisposed between the gate electrode and the semiconductor layer.